Tom Brady is helped off the field after his injury. Photograph: Elsa/Getty Images

The best laid plans of mice and men may often go awry, but rarely has an NFL season started with as momentous a twist as this. When the New England Patriots quarterback, Tom Brady, fell awkwardly under a hit from the Kansas City Chiefs safety Bernard Pollard midway through the first quarter of their season opener, his piercing scream stunned Gillette Stadium into silence. When Brady failed to return to his feet, last season's AFC champions knew their whole season had been redefined.

Initially the Patriots suggested Brady could return before the end of the game, but as the afternoon wore on it became increasingly apparent that he would do well to make it back before the end of the year. The Patriots head coach, Bill Belichick, is notoriously coy when it comes to divulging the truth about his players' injuries. Indeed, there was irony in the fact Brady should suffer his injury in the first game since 2005 when he has not been listed on the team's pre-game injury report with a mystery "shoulder problem" that never caused him to miss a start. But the New York Jets quarterback, Brett Favre, said after his team's 20-14 win over the Miami Dolphins that word had reached him of Brady tearing a cruciate ligament. Soon after a number of media outlets reported that Brady had torn the ACL in his left knee.

"Patriots fans, welcome to your worst nightmare," groans today's Boston Globe, beside headlines asking: "Are the Pats finished?" and "Can Belichick win without Brady?" NFL.com's senior columnist Vic Carucci believes they can't, arguing that without Brady the Patriots' hopes of returning to the Super Bowl come to a "screeching halt". ESPN.com's Gene Wojciechowski goes as far as to suggest the New York Jets may even be the new favourites to win the AFC East.

There is exaggeration here, for sure – not least in the assessment of a Jets team who were no better than solid yesterday – but it is fair to say that without Brady the Patriots are a totally different prospect. Brady is a four-time Pro Bowler and former league MVP who has won three Super Bowls and last year set the record for touchdowns in a single season. His back-up Matt Cassel is a fourth-year pro who hasn't started a competitive game at quarterback since high school.

Identified by ESPN as one of the top 100 prospects coming out of high school in 1999, Cassel was unfortunate to find himself behind Heisman trophy winners Carson Palmer - now of the Cincinnati Bengals – then Matt Leinart – now of the Arizona Cardinals – during his time at the University of Southern California. In the end he started games only as a tight end, H-back and special teams player, and many were surprised that he was drafted at all (the Patriots took him with the 230th pick of the 2005 draft). How many other NFL quarterbacks can honestly say that their favourite moment as a college player was recovering an onside kick?

To be fair to Cassel he performed capably yesterday - completing his first pass to Randy Moss for 51 yards before completing his first drive with a 10-yard scoring toss to the same player and eventually leading the Pats to a 17-10 win. But the Chiefs, who themselves lost starting quarterback Brodie Croyle during the third quarter, were hardly stiff opposition, and besides, "capable" would represent a huge drop-off from Brady.

If Brady is indeed out for the season the Patriots will likely add another veteran at quarterback, and perhaps even try to lure Moss's former team-mate Daunte Culpepper out of his recent retirement, but even then such a player would need time to learn the playbook. Cassel looks likely to start next week, at the very least, making the Patriots' trip to play Favre's Jets in New York quite the prospect.

Favre was everything the Jets wanted him to be on his first start – confident, entertaining and utterly determined to open with a win. The Jets were left without a place-kicker when Mike Nugent hurt his thigh in the first quarter, but that only left more room for classic Favre improvisation – forcing New York to take a shot at the end zone on fourth-and-13 at the Dolphins' 22 later in the half. With a defender hanging off his shoulder Favre heaved a fluttering jump ball towards the end zone. By rights it should have been broken up. Somehow you knew it wouldn't be.

To say that Favre was everything Chad Pennington hasn't been these past few years for the Jets would be unfair. Pennington actually completed more passes for more yards than Favre yesterday despite having worse receivers and little help from the Dolphins' running game, and showed off his own moxie by flipping an expertly weighted 11-yard touchdown pass to David Martin despite being hit by defensive end Calvin Pace as he threw.

That said, the difference in the two players' arm strength is there for all to see. Favre's first touchdown throw – a 56-yard strike to Jerricho Cotchery - was longer than all but one of the Jets' passes last year.

But even if Favre's debut and Brady's injury has allowed the Jets to dream of an upset next weekend, it is hard to envision them challenging in the AFC. Then again the two favourites to challenge New England before the season both lost their openers yesterday. The Indianapolis Colts looked particularly disjointed as they slumped 29-13 at home to Chicago, while the San Diego Chargers recovered from 19-10 down early in the fourth quarter to lead the Carolina Panthers 24-19, only to then give up a touchdown on the last play of the game and lose.

The Pittsburgh Steelers on the other hand, defied their pre-season doubters (like, er, me) by blowing away a Houston Texans team that was supposedly on the rise. Their performance yesterday suggested they could yet emerge as the AFC's top seed, but they still have a brutal schedule ahead of them.

Waiting for Ocho

In the end, Ocho Cinco never did show up for Cincinnati's trip to Baltimore. The NFL confirmed on Thursday that it had officially recognised Bengals' wide receiver Chad Johnson's legal name change to Chad Javon Ocho Cinco, but Cincy announced 15 minutes before kick off that "certain issues" needed to be resolved with the league before he could wear the name on his shirt. Sadly for them Johnson didn't bother to show up either – catching just one pass in a 17-10 defeat.

Bums in seats

For the fourth year running, every game sold out on opening weekend. That might not sound a big deal in the world's best-attended league, but it certainly is for all the fans who can't make it down to their local stadium on Sunday, since the NFL's blackout rule means matches that aren't sold out 72 hours before kick-off cannot be broadcast within 75 miles of the stadium where they are played.

Three and out

• The Dallas Cowboys looked ominously good as they rolled to a 28-10 win over the Browns in Cleveland, moving the ball at will on offence. Quarterback Tony Romo was not sacked once in 32 attempts and the only cause for concern for Dallas should be that he was still picked off once and could have been more times despite the total lack of pressure.

• Once again Vince Young struggled, yet once again the Tennessee Titans opened the season with a win over the Jacksonville Jaguars. Sometimes, in this league, a solid running game and a stubborn defence really is enough. The Jags should be worried – David Garrard only threw three interceptions in total last season. Yesterday he threw two.

• Two rookie quarterbacks started yesterday under rookie head coaches, and both won. Atlanta's Matt Ryan became the first player since Michael Bishop to complete his first NFL attempt for a touchdown, while Baltimore's Joe Flacco went in on a 38-yard scoring run, but both got plenty of help along the way. Flacco was aided by an offensive line that didn't let him get sacked once by Cincinnati. Ryan's biggest help came from running back Michael Turner, who dashed for 220 yards and two scores. And from a Detroit Lions defence that has clearly learnt nothing since giving up 6,042 total yards and 444 points last year.